1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming system and a recording medium.
2. Description of Related Art
Image forming apparatuses (printer, copier, facsimile or the like) using an electrophotographic process technique generally form an electrostatic latent image by irradiating (exposing) a charged photoconductor drum (image carrier) with laser light based on image data. Toner is supplied from a developing apparatus to the photoconductor drum on which the electrostatic latent image is formed, the electrostatic latent image is thereby visualized and a toner image is formed. Furthermore, the toner image is directly or indirectly transferred to a sheet, heated, pressurized and fixed by a fixing nip, and a toner image is thereby formed on the sheet.
When printing is done on continuous sheet (sheet) using such image forming apparatuses, base printing may be followed by overprinting whereby printing is further done thereon. Overprinting is done in two modes; one in which printing is done on all continuous sheet once and printing is then done on the continuous sheet over again from the beginning, and the other in which printing is stopped halfway, fixing and transfer are cancelled once, and continuous sheet is rewound up to a position where overprinting starts, then the continuous sheet is sent again for second printing. Here, from the standpoint of labor-saving, the mode of rewinding continuous sheet is more desirable.
For example, when base printing is done on only several centimeters of several hundreds of meters of continuous sheet, rewinding may improve work efficiency because it is possible to reduce time and effort required for printing several hundreds of meters of continuous sheet. On the other hand, in the mode in which rewinding is performed, continuous sheet is rewound and a fixing process is performed twice, and it is thereby possible to improve glossiness of continuous sheet.
For example, Japanese Patent Application Laid-Open No. 2010-97132 discloses a configuration in which rewinding is performed once when a suspended printing process is resumed so as not to produce any excessive blank portion in continuous sheet. In the configuration disclosed in Japanese Patent Application Laid-Open No. 2010-97132, when the printing process is resumed, it is possible to synchronize the position of a toner image on the continuous sheet with the transfer, fixing and pressure-contacting positions through rewinding.
However, since the amount of water of the continuous sheet that has once passed through the fixing portion is changed by heating and the continuous sheet contracts, when the toner image region is arranged biased to one side in the width direction of the continuous sheet during base printing, one side of the continuous sheet contracts, which causes partial distortion on the continuous sheet. For that reason, if the continuous sheet is rewound, resent and fixed in such a case, wrinkles are produced in the continuous sheet. With the continuous sheet in particular, such a problem is likely to occur because the toner image often has the same pattern in the conveyance direction.